glamor_fds_from_pixmap returns 0 on error, but we were treating that as
success, continuing with uninitialized stride and fd values.
Also bail if the offset isn't 0, same as in dri3_fd_from_pixmap.
v2:
* Reduce to a simple one-liner fix (Emil Velikov)
Fixes: c8c276c956 "glamor: Implement PixmapFromBuffers and
BuffersFromPixmap"
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
(cherry picked from commit 4d5950ce14)